Varistors are often used to protect the flow of increased current in the presence of excess voltage. As will be understood, a varistor is a voltage-dependent resistor. As such, varistors are often included in a circuit to shunt current created by high voltage away from sensitive components.
For example, a varistor may be used in power supplies that are supplied by AC mains (e.g. 110 or 220 volts AC). The varistor is usually placed downstream of the power supply fuse, between the AC mains live conductor and neutral. In the presence of a high transient voltage, the varistor clamps the voltage and shunts any resulting current.
A typical varistor is formed of a bulk material (such as ceramic) between two conducting plates. The bulk material contains grains of a conducting material (such as small amount of zinc, bismuth, cobalt or manganese) which acts to form diode junctions that allow current to flow only in one direction in the presence of a moderate voltage. Only small currents flow through the diode junctions, caused by reverse leakage. The presence of a large applied voltage, on the other hand, causes the diode junctions to break down and the varistor to conduct.
Unfortunately, after repeated breakdowns caused by applied high voltages the varistor may fail. Such a failure may be catastrophic to downstream components, and the varistor itself.
Accordingly, varistor circuits that allow for the prevention and/or detection of possible catastrophic failures are desirable.